Generally, an active matrix type liquid crystal display device has a structure of a pair of glass substrates opposed and fixed which between a liquid crystal is filled in a space. More specifically, transparent common electrodes are provided on one glass substrate, a large number of transparent pixel electrodes are provided in a matrix manner on the other glass substrate, and a circuit for individualistically applying voltages to the pixel electrodes.
The liquid crystal display device performs display operation with sandwiched polarizing plates in the foregoing structure, so that it has a characteristic of a narrow viewing angle.
To increase a viewing angle, have been proposed liquid crystal display devices utilizing IPS (In Plane Switching), MVA (Multi domain Vertical Aline), or ASV (Advance Super View) mode as a physical technique such as divided orientation.
Here, the following will explain a typical technique for increasing a viewing angle.
First, referring to FIG. 27, TN (Twisted Nematic) mode will be explained as follows. In FIG. 27, bold and black lines represent crystal elements.
FIG. 27 shows the movement of the liquid crystal elements in the TN mode. When no voltage is applied (voltage is OFF), the liquid crystal elements are oriented as shown on the left in the drawing. As a voltage is applied, the liquid crystal elements are caused to stand as shown in the middle of the drawing. When a maximum voltage is applied, the liquid crystal elements are oriented as shown on the right in the drawing. Each gradation level is expressed by change in applied voltages.
In the foregoing TN mode, the liquid crystal elements are oriented obliquely and a viewing angle characteristic occurs depending on the oriented direction. Here, the occurrence of the viewing angle characteristic means the state in which display image cannot appear normally depending on the angle at which a display screen is viewed.
Such a viewing angle characteristic occurs because the liquid crystal elements have a bar shape and a polarization characteristic. More specifically, when a voltage is applied, the liquid crystal elements, each of which has the same characteristic, move in the same direction. This causes the viewing angle characteristic with respect to the leaning angles of the liquid crystal elements.
Conventionally, to reduce the effects by the polarization characteristic of the liquid crystal, an orientation dividing method is adopted as shown in FIGS. 28(a) and 28(b). Unlike the usual orientation, the method reduces the polarization characteristic by dividing the orientation of pixels in the different orientation directions so as to disperse the orientation directions of the liquid crystal.
This orientation dividing method does not cause the viewing angle characteristic of the liquid crystal elements in the TN mode, so that it is possible to realize the increased viewing angle.
Secondly, referring to FIGS. 29(a) and 29(b), the IPS (In Plane Switching) mode will be explained as follows.
In the IPS mode, as shown in FIG. 29(a), the longitudinal direction of the liquid crystal elements is in parallel to the panel plane, so that although the IPS mode has a low dependency on a physical viewing angle, it has a wavelength dependency on light that transmits the liquid crystal element, and the amount of this wavelength dependency causes change in the viewing angle. Also, human eyes have a wavelength characteristic, so that the wavelength dependency changes in luminance on the display screen. This causes the problem of a narrow viewing angle.
Conventionally, to realize an increased viewing angle, proposed has been a method of dividing the orientation in zigzag so as to cancel the wavelength dependency (super IPS).
Note that, the IPS mode has two major disadvantages:
(1) Response speed is low; and
(2) Transmittance is extremely poor.
Next, referring to FIG. 30, the VA (Vertical Alignment) mode will be explained as follows.
In the VA mode, as shown in FIG. 30, when no voltage is applied (OFF), the longitudinal direction of the liquid crystal elements is vertical to the panel plane. When a voltage is applied (ON), the longitudinal direction of the liquid crystal elements is horizontal to the panel plane. Therefore, viewing angle characteristic improves when a voltage is ON and OFF. Incidentally, in a halftone at which a mediate voltage is applied, the liquid crystal elements are oriented obliquely in one direction, which causes the viewing angle characteristic. The viewing angle characteristic in this case is in the same level as that of the TN mode.
Thus, in the VA mode, the viewing angle characteristic occurs in the halftone, which results in the problem of the narrow viewing angle.
Note that, as compared with the IPS mode, the VA mode has the following characteristics:
(1) Response speed is high;
(2) Contrast can be gained because black level is high in quality; and
(3) Transmittance is better than that of the IPS mode, although it is worse than that of the TN mode.
To improve the viewing angle characteristic of the halftone in the VA mode, the following MVA (Multi-domain VA) mode has been proposed.
Next, referring to FIGS. 31(a) and 31(b), the MVA mode will be explained as follows.
The MVA mode is the mode in which the VA mode is subjected to orientation division. Such an orientation division can improve the viewing angle characteristic of the halftone.
More specifically, as shown in FIG. 31(a), an object having a structure of a substantially triangular shape on cross section is applied onto the panel plane, and oriented films are further formed thereon. Therefore, as shown in FIG. 31(b), because of the foregoing object on the panel plane, the liquid crystal elements lean along the object when a voltage is applied, which produces the effects of divided orientation in the halftone. In such a manner, an increased viewing angle is realized in the VA mode.
Note that, the VA mode can improve the viewing angle characteristic by the divided orientation as described above; however, it does not improve so much as the IPS mode.
Furthermore, unlike the foregoing physical method such as the divided orientation, Japanese Laid-Open Patent Publication No. 121144/1995 (Tokukaihei 7-121144; published on May 12, 1995) (a Japanese equivalent to the U.S. Pat. No. 5,847,688) proposes a liquid crystal display device in which a viewing angle is electrically increased, utilizing a plurality of different gamma characteristics based on an input image signal.
Incidentally, the width of the viewing angle in the liquid crystal display device is defined by the width of the area where a contrast ratio of white to black is not less than a predetermined value. Note that, a gradation curve is also an important element for the accurate reproduction of images. Since the gradation curve does not significantly change depending on the viewing angle in display devices, except for a liquid crystal display device, such as a cathode-ray tube monitor and a plasma monitor, the definition of the width of the viewing angle is usually considered to be no problem.
However, the gradation curve is an important element for the reproduction of images. For example, in a display device of 256 gradation levels, the gradation curve when viewed from the front has:
a luminance ratio=(n/255)2.2, and
the gradation curve when viewed from the side has:
a luminance ratio=(n/255)1.0, where “n” is a gradation.
At this moment, in case of the display of a gray color with gradation level 128, the gradation level 128 is displayed when viewed from the front. On the other hand, a gray color with gradation level 186 is displayed when viewed from the side, and a whitish display is made as compared with when viewed from the front.
Further, in case where the gradations of R, G, B are different, the difference in gradation display is remarkable. For example, when R is at gradation level 0, G is at gradation level 128, and B is at gradation level 255, the luminance ratio of the front is R:G:B=0:0.22:1. On the other hand, the luminance ratio of the side is R:G:B=0:0.50:1, which indicates the change into a strongly green-tinged display.
As described above, even the same original data is changed into different image depending on the change in the gradation curve.
Therefore, in view of a contrast ratio, the liquid crystal display devices utilizing the wide viewing angle modes such as ISP, MVA, and ASV modes realize a wide viewing angle. However, the gradation curve is different when viewed from the side. This means a lack of image reproduction when viewed from the side.
Thus, the difference in the gradation curve between when viewed from the front and from the side is referred to as distortion of gradation curve.
Further, the liquid crystal display device disclosed in the foregoing publication increases the viewing angle by improving the viewing angle characteristic when viewed from the side, using gamma characteristic, so that the gradation curve when viewed from the front distorts. Especially, in case where the viewing angle characteristics at the both sides sandwiching the front deviates in the same direction as that of the target gamma characteristic, it is necessary to largely change the gradation curve when viewed from the front.
This means to bring about the deterioration of the image reproduction when viewed from the front.
As described above, in all of the conventional liquid crystal display devices realizing an increased viewing angle, the gradation curve when viewed from the front is different from that when viewed from the side, in other words, the gradation curve's distortion with respect to the viewing angle occurs in the display image, so that image when viewed from the front is different from that when viewed from the side. As a result, it is impossible to obtain an excellent quality of image in a wide range of viewing angle, which causes the problem of the deterioration in a display quality level.
Further, the conventional liquid crystal display device has a constant range of viewing angle, it is necessary to replace the display deice itself when change in the range of viewing angle is desired as in the case when information desired to be shown to other people is arranged so as to be shown to many other people and the case when information not desired to be shown to other people is arranged so as not to be shown.